1. Field of the Invention
The present invention relates to a method for forming a reflection-type light diffuser, and more particularly, to a method for forming a reflection-type light diffuser applied to a reflective liquid crystal display (LCD).
2. Description of the Prior Art
Generally, there are two basic types of LCDs according to image display modes: a light transmissive type and a light reflective type. The light transmissive LCD comprises a backlight disposed on a rear side of a liquid crystal cell for emitting light. The light radiated from the backlight selectively passes through the liquid crystal cell, thereby realizing desired images. The light reflective LCD comprises a front light source and a reflective plate disposed on a rear side of the LCD so as to reflect incident light generated from the front light source toward the front side of the LCD, thereby realizing desired images. The users can choose the light transmissive LCD or the light reflective LCD of their own accord.
Because the prior art reflective plate of the light reflective LCD reflects the incident light towards a fixed point of view, the LCD has a narrow field of vision that restricts users to viewing displayed images on the LCD to angles around a specific angular magnitude (i.e. a visible angle). Therefore, for the purpose of increasing the visible angle of the light reflective LCD, a plurality of bump structures are formed on the reflective plate to rough the surface of the reflective plate and increase scattering angles of the reflected lights and thus broaden the viewable angle of the light reflective LCD.
Please refer to FIG. 1A to FIG. 1D of schematic diagrams illustrating a prior art method for forming a reflection-type light diffuser 28 on a glass substrate 10. As shown in FIG. 1A, a spin-coating process is performed to form a resin material layer 12 on the glass substrate 10, and the resin material layer 12 is pre-baked for about 30 minutes at about 300° C. The glass substrate 10 has a thickness of about 1.1 centimeters (cm), and the resin material layer 12 has a thickness of about 1.5 micrometers (μm). Then as shown in FIG. 1B, an exposing process is performed by using a photo mask 14 and a light source 20. The photo mask 14 has a plurality of light-shielding regions 16 and a plurality of light-transmitting regions 18, so that a photoresist pattern 22 is formed in the resin material layer 12 after a subsequent developing process, as shown in FIG. 1C. As shown in FIG. 1D, a soft baking process is performed to soften the photoresist pattern 22 to form a continuous photoresist pattern 24. A metal layer 26 is formed on the glass substrate 10 and the reflection-type light diffuser 28 is completed. The metal layer 26 comprises aluminum (Al), nickel (Ni), chromium (Cr), or silver (Ag) metal, and the metal layer 26 has a thickness of between 0.01 to 1.0 μm. The photoresist pattern 24 comprises a plurality of bump structures 24.
Because the reflection-type light diffuser 28 has a plurality of bump structures 24, the surface of the reflection-type light diffuser 28 is rough and uneven. When incident light 30 enters the reflection-type light diffuser 28, the incident light 30 generates a plurality of scattering lights 32 by reflecting from the metal layer 26 and the bump structures 24. However, the bump structures 24 are disposed on the reflection-type light diffuser 28 randomly, so that the scattering directions of the scattering lights are too wide and thus result in weakening the scattering light 32 intensity. In addition, the scattering lights 32 also interfere with each other. Therefore, another reflection-type light diffuser is disclosed to solve the above-mentioned problems.
Please refer to FIG. 2A to FIG. 2F of schematic diagrams illustrating a prior art method for forming a reflection-type light diffuser 52 on a glass substrate 40. As shown in FIG. 2A, a resin material layer 42 is spin-coated on the glass substrate 40, and the glass substrate 40 is pre-baked. As shown in FIG. 2B, an exposing and developing process is performed by using a photo mask 44 to form a photoresist pattern 46 in the resin material layer 42. The photoresist pattern 46 comprises a plurality of straight protrusions, as shown in FIG. 2C. Then as shown in FIG. 2D, the glass substrate 40 is turned over 90 degrees and thus tilted toward a perpendicular direction. A thermal treatment process is performed to soften the photoresist pattern 46. Since the photoresist pattern 46 is made of heated plastic material, the photoresist pattern 46 can be heated and softened to form a photoresist pattern 48 using gravity. The photoresist pattern 48 includes asymmetrical silt structures as shown in FIG. 2E. Finally, as shown in FIG. 2F, the glass substrate 40 is put back to its original horizontal position and then cooled down. A metal layer 50 is formed on the glass substrate 40 and the reflection-type light diffuser 52 is completed.
When incident light 54 enters the reflection-type light diffuser 52, the incident light 54 generates a plurality of scattering lights 56 by reflecting from the metal layer 50 and the straight protrusions 48. However, the reflection-type light diffuser 52 with the straight protrusions 48 still has a problem of the light directionality. Therefore, the U.S. Pat. No. 6,163,405 discloses a reflection-type light diffuser to solve the problems of light scattering and light direction. Please refer to FIG. 3 of a schematic diagram illustrating a prior art reflection-type light diffuser 68. As shown in FIG. 3, a photoresist pattern 62 and a metal layer (not shown in FIG. 3) are formed on a glass substrate 60, and the photoresist pattern 62 comprises a plurality of parallel slant structures 64 and a plurality of knob structures 66 disposed on the slant structures 64. A multi-exposure shift process is utilized to form the slant structures 64 by using a photo mask (not shown in the FIG. 3). The photo mask is moved at a fixed distance many times and the exposure processes with different exposure powers are performed to form the slant structures 64 in the multi-exposure shift process. And then another photo mask (not shown in the FIG. 3) is utilized to form the knob structures 66 on the slant structures 64. Although the reflection-type light diffuser 68 with the knob on slant structures improves the above-mentioned problems, the structures are fabricated by using the exposure processes many times and the two different photo masks to form the knob on slant structures, resulting in complicating the process and increasing costs.